1. Field of the Invention
This invention relates generally to projection type liquid crystal systems such as projection computer displays, projection monitors, projection video or television systems, and more particularly to a low cost polarization conversion system and the optical arrangement for a projection type liquid crystal system based on polarization modulating liquid crystal displays.
2. Description of the Related Art
Polarization conversion systems for use in projection type liquid crystal displays (LCDs) are well known in the prior art. In a projector, light from a source such as a metal halide arc lamp is collected by a reflector and relayed onto an object (e.g., a LCD light valve), with the help of condenser and relay lenses. The illuminated object is then projected onto a display screen at a desired magnification. However, since a dominant type of liquid crystal displays form the image by discriminating between the polarization of light from bright and dark pixels, the light collected from the light source must be polarized by means such as a linear polarizer before being incident on the liquid crystal light valve. In the light valve, the bright and dark pixels are discriminated by the polarization of the light leaving them. This light is then analyzed by means of an analyzer. Typically, the act of polarization of unpolarized light by the conventional polarizers implies that the light of unwanted polarization be lost due to absorption in the polarizers. The alternative of using polarizing cube or plate beam splitters results in transmitting one polarization and reflecting the other. In such a situation, the unused light of unwanted polarization must be recaptured and its polarization altered to be that of the used polarization before it can be used to illuminate the light valve. These polarization conversion systems generally comprise a light reflector, a quarter wave plate and/or a half wave plate. A quarter wave plate shifts the polarization of incident light by 45 degrees. Similarly, a half wave plate shifts the polarization of incident light by 90 degrees.
In U.S. Pat. No. 5,200,843 to Karasawa et al., the polarization and polarization conversion takes place over the full aperture of the light source and therefore the outgoing light must go through a intensity homogenizer before it is incident upon the light valve. While illumination systems of this type have their benefits, they tend to be bulky and expensive.
Still other polarization conversion systems are known in the prior art, such as those disclosed in EP A1 0753780 and EP A1 0753971, shown in FIG. 1A, and referred to generally as reference numeral 100. In these systems the full aperture of the light coming from a light source 101 and a reflector 102 which reflects the light from the light source in the direction indicated by arrows A, is sampled, subdivided, and focused with the help of an array of lenses 103, resulting in an array of beam samples 114. Near the focus of these beam samples is the polarization conversion system 120 which consists of a stack of plate polarizing beam splitters 115, each plate of the stack seeing one row of beam samples 114. Since the polarization conversion system 120 is in the vicinity of the focus of the beam samples, the beam samples are separated from each other and a copy of the beam sample can be placed at its side. For example, the beam sample 114 is separated into components of two polarizations with the P-polarization 117 being transmitted and the S-polarization 116 being reflected from the front side 115a of the polarizing beam splitter 115. The S-polarization is further reflected by the rear side 115b of the plate polarizing beam splitter 115. The reflected beam then goes through a half wave plate 130 where its polarization is shifted 90xc2x0 to become a P-polarization 118 upon transmission from the plate. Thus the outgoing beam has suffered minimal amplitude loss and has acquired a single type of linear polarization in the form of beams 117 and 118. This polarization conversion occurs for all the beam samples which are then redirected towards the light valve (not shown) by a second array of lenses 113. The appropriately magnified and overlapping beam samples illuminate the light valve with polarized light in a very efficient way. The side of the polarization conversion system 120 of FIG. 1A facing the reflector has the alternating plates of the stack blocked in the form of light blocks 122 to maximize polarization purity. The system has many components and while the assembly is compact and efficient, it is relatively expensive to manufacture such polarization conversion systems 120.
FIG. 1B shows an enlarged schematic view of the polarization conversion system 120 of FIG. 1A. It consists of a stack of plane parallel plate polarizing beam splitters 115 with one side of the plate 115a having a polarizing beam splitting multi-layer dielectric coating and the other side 115b being blocked by the light blocking grid 122 so it does not receive any direct light. As shown, the light is incident on alternate plates only. The grid of half wave plates 130 is similarly applied to alternate plates so the half wave plate 130 only sees the reflected light 116a of S-polarization 116 and converts it to P-polarization 118. While this arrangement has its advantages the assembly and alignment of all the components is expensive. Also, the multi-layer thin film polarizing films only work over a limited range of angles.
For the above reasons, there is a need in the art for a simple, low cost polarization conversion system which is capable of operation at high numerical apertures.
Therefore, it is an object of the present invention to provide a polarization conversion system which produces linearly polarized light from unpolarized light without losing half the light intensity.
Another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light which enables a brighter image without increasing the wattage of the light source.
Yet another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light for use in projection displays.
Yet another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light which is of simpler construction and thus more economical.
Yet still another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light which performs over a broad range of incident light angles (i.e., numerical apertures).
Yet another object of the present invention is to provide a polarization conversion system which produces linearly polarized light which performs over a broad range of wavelengths covering the three primary colors, red, green and blue.
Accordingly, a polarization conversion system for converting incident light is provided in which the incident light having at least a first and second polarization is converted to light of one of the first and second polarizations. The polarization conversion system comprises an input side at which the incident light enters and an output side at which light of one of the first and second polarizations exits. A polymer based reflective polarizing material disposed between the input and the output sides, in communication with the incident light, and angled with respect to the incident light for transmitting light of one of the first and second polarizations and reflecting the other is provided. A reflector for reflecting the reflected light from the polymer based reflective polarizing material and a light block disposed on the input side for blocking the incident light from communication with the reflective means are also provided. Lastly, a polarization convertor for shifting the polarization of the reflected light to that of the transmitted light is provided. The polymer based reflective polarizing material, the reflector, the polarization convertor, and the light block are arranged such that the transmitted light of one of the first and second polarizations exits the output side, and the reflected light is directed towards the reflector which directs it towards the polarization convertor which in turn shifts the polarization of the reflected light to that of the transmitted light before exiting the output side.
Another aspect of the present invention are polarization modulating liquid crystal projection display systems utilizing the polarization conversion systems of the present invention.
It can be realized that while the invention is described relative to a parabolic reflector that produces a largely collimated beam of light, it may also be readily employed with elliptical or other types of reflectors by incorporation of collimating condenser lens to produce a collimated beam as is known in the art.